Comparison of the Somatotropic Axis in Free-ranging and Rehabilitated Harbor Seal Pups (Phoca vitulina)

Abstract

The somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF binding proteins (IGFBP), is a bridge between growth physiology, developmental age, and nutritional status in domestic animals. However, the importance of the somatotropic axis in nutrition, growth, and development of harbor seals has not been previously explored. Given the difficulty of conducting longitudinal studies in free-ranging harbor seals, this study focused on the potential use of harbor seals in rehabilitation facilities as a model for free-ranging seals. The purpose of this research was to compare concentrations of components of the somatotropic axis in free-ranging versus rehabilitated harbor seal pups. The hypothesis was that measurements of the somatotropic axis will be similar between individuals of comparable age and nutritional status (fasting versus feeding). To investigate this hypothesis, harbor seal pups (n=8) brought to The Marine Mammal Center (Sausalito, California, USA) or Mystic Aquarium (Mystic, Connecticut, U.S.A.) were initially assessed and determined to be healthy but abandoned. All pups were less than 2 wk of age upon arrival at rehabilitation facilities. Standard length was assessed at the time of arrival and again at release. Body mass was measured every week and blood samples were collected from each pup at 0, 4, and 8 wk of rehabilitation. Blood was collected and morphometrics assessed in free-ranging harbor seal pups (n=8) from the Gulf of Maine. Sera were analyzed for GH, IGF-I, and IGFBP concentrations. Concentrations of GH, IGF-I, and IGFBP-2 and -3 in rehabilitated pups were within a similar range compared with free-ranging pups when considered in the context of presumed nutrient intake. These data suggest that rehabilitated harbor seals may provide a useful model to investigate the effects of nutrient intake on growth and development of harbor seals, and will provide insight into phocid endocrinology and metabolism.

Seasonal influence on the response of the somatotropic axis to nutrient restriction and re‐alimentation in captive Steller sea lions (Eumetopias jubatus)

“…Specifically, northern elephant seal and harbor seal pups exhibit increased GH and decreased IGF-I concentrations while fasting that are associated with increased utilization of adipose stores to meet energy needs (Ortiz et al, 2003;Richmond et al, 2010). Increased nutrient intake stimulates an increase in IGF-I concentrations in both adult female Weddell seals and harbor seal pups and is associated with accumulation of lean and adipose tissue (Eisert et al 2005;Richmond et al, 2008Richmond et al, , 2010. However, the nature of the physiological response to nutritional stress is not constant, and depends at least partly on concurrent life history demands.…”

“…During limited nutrient availability, IGFBP-2 is increased, reducing availability of IGF-I for peripheral tissues (Renaville et al, 2000). Investigations of the response of the somatotropic axis to changes in nutrient intake in marine mammals has been limited, but studies with phocid (true seal) species suggest that they respond in a similar manner as terrestrial mammals when intake is restricted or increased (Ortiz et al, 2003;Eisert et al, 2005;Richmond et al, 2008).…”

Response of metabolic hormones and blood metabolites to realimentation in rehabilitated harbor seal (Phoca vitulina) pups

“…In this study, as expected, GH concentrations were greatest at nutritional nadir (fasting, rehabilitation week 0) and significantly decrease upon realimentation. However, despite this dramatic decrease, by the end of the 10-week realimentation period, concentrations still remained greater than well-fed wild harbor seals of comparable age (approximately 2-4 ng/ml; Richmond et al 2008). As expected, IGF-I increased during realimentation; however, precise determination of differences between nutritional nadir and early realimentation was limited by the detection limits of the assay (40 ng/ml).…”

Evaluation of an alternative to feeding whole frozen fish in belugas (Delphinapterus leucas)

“…As inclusion rate of Fish Analog increased, GH concentration increased and IGF‐I decreased. Typically, this pattern of response is indicative of reduced nutrient intake or nutritional stress [Thissen et al, 1994; Richmond et al, 2008]. Although the overall changes in GH and IGF‐I observed with increasing inclusion may suggest that the animals metabolic needs were not met, alternatively, these data may reflect the normal pattern of hormonal change owing to seasonal changes in intake [Richmond et al, in preparation].…”